OF CONOHOLOQY. 135 



PachycJdli. There is in this group also a marked distinct- 

 ness of form. As we have excluded this genus from the 

 family Strepomatidse on considerations entirely conchological, 

 it is very interesting to find in the dentition dift'ereuces quite 

 as marked as those existing in the shell. To show the very 

 peculiar form of the Rhachidian tooth, we copy from Troschel 

 the following for comparison : — 



Pachychilus Isevissimus. See plate 12, fig, 3. 

 " Schiedeanus. " 11, " 4. 



It is curious, however, and shows how little dependence can 

 be placed on any one character in the grouping of Mollusca, 

 to find Pirena and Melanopsis placed by this author together 

 with Pachychilus^ on account of their almost identical denti- 

 tion, when they differ so much in conchological characters and 

 in geographical distribution. 



Dr. VYilliam Stimpson, nearly two years since, published a 

 paper in the American Journal of Science and Arts, " On the 

 Structural Characters of the so-called Melanians of North 

 America," containing the results of observations of the ani- 

 mals of several of our species, including an /o, Anculosa, and 

 Ooniohasis. The individuals of these three very distinct 

 genera were not found to differ one from another in any struc- 

 tural character, although readily distinguished from Oriental 

 species. We will state the differences in their relative im- 

 portance as they appear to us. 1st. By being oviparous, 

 while the latter are ovo-viviparous. 2d. By the mantle mar- 

 gin being plain in the American, and fringed in the exotic 

 family. 3d. By difference in dentition. To these may be 

 added a sufficient conchological difference to justify the sepa- 

 ration into two families, even if the soft parts were undistin- . 

 guishable. 



